Grain size of metal castings

ABSTRACT

METHOD OF PRODUCING A FINE-GRAINED OR CHANGED GRAIN SIZE ALLOY CASTING. A DEFINE RELATIONSHIP IS DISCLOSED BETWEEN CAST GRAIN SIZE AND A PARAMETER WHICH IS OBTAINABLE FROM EXISTING PHASE DIAGRAM INFORMATION. THIS PARAMETER IS   -(ML) (1-KO)CO P= KO   WHEREIN ML IS THE LIQUIDUS LINE SLOPE, KO IS THE SOLUTE DISTRIBUTION COEFFICIENT, AND CO IS THE BULK COMPOSITION IN TERMS OF ATOMIC PERCENT OF THE GRAIN SIZE REFINING ADDITIVE.

April 3, 1973 L. A. TARSHIS ET AL 3,725,057

GRAIN SIZE OF METAL CASTINGS 3 Sheets-Sheet 2 Filed Sept. 21, 1971 X ATOM/C PERCENTALLOYADD/T/ON I00 ATOM/C PERCE/V7'8HSE METAL April 3, 1973 I F iled Sept. 21, 1971 ll-w RELATIVE GRAIN SIZE 9 D Q A q, Q

| A. TARSHIS ET AL 3,725,057

GRAIN SIZE OF METAL CASTINGS 3 Sheets-Sheet 3 PUREA/ Fig.3.

l 1 l l I in van for-s: Lemuel A.7'a1-sh/s, dame: L... Walker-j,

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United States Patent 3,725,057 GRAIN SIZE OF METAL CASTINGS Lemuel A. Tarshis, Latham, and James L. Walker,

Schenectady, N.Y., assignors to General Electric Comp y Continuation-impart of application Ser. No. 882,512, Dec. 5, 1969. This application Sept. 21, 1971, Ser. No. 182,406

Int. Cl. C22c 1/02 US. Cl. 75-170 6 Claims ABSTRACT OF THE DISCLOSURE Method of producing a fine-grained or changed grain size alloy casting. A definite relationship is disclosed between cast grain size and a parameter which is obtainable from existing phase diagram information. This parameter m.) 1K. c.

wherein m;, is the liquidus line slope, K is the solute distribution coefiicient, and C is the bulk composition in terms of atomic percent of the grain size refining additive.

The invention herein described was made in the course of or under a contract or subcontract thereunder with the Department of the Army.

This application is a continuation-in-part of copending application Ser. No. 882,512, filed Dec. 5, 1969, in the names of Lemuel A, Tarshis and James L. Walker entitled Grain Size of Metal Castings, and assigned to the same assignee as the present application.

This invention relates to the as-cast grain size of metals and more particularly to the provision of a method for producing a preselected change in the grain size of castings by the addition of small amounts of grain refining materials or elimination of specific impurities.

More specifically, it has been found that a definite relationship exists between the structural characteristics, namely grain size, of metal castings and a parameter which utilizes existing, well-known phase diagram information. This permits the modification of grain size in a given casting by the addition of relatively small amounts of appropriate alloying agents on the basis of readily available, published phase diagram information. In general, the process of the invention involves the concept that the nature of solute redistribution in castings controls the resulting grain size and is related in a definite manner to the bulk composition of the solute, the solute distribution coeflicient and the phase diagram liquidus line slope, all of which are obtainable from known, published data, particularly with respect to binary alloy systems. In more complex system, individual solute elements can control as-cast grain size and the binary criteria based upon the phase diagram information of the major constituent and the grain-size controlling additive may be applied according to a parametric relationship to be subsequently discussed in detail.

In order to more fully disclose the invention, the follow ing detailed description of the invention will be made with reference to the accompanying drawings in which FIG. 1 is a graph illustrating the measured relationship of the as-cast grain size of nickel alloyed with one atomic percent of various elements to the mentioned parameter;

FIG. 2 is a portion of a hypothetical phase diagram of a binary system;

FIG. 3 is a graph similar to FIG. 1 in which the solvent metal is aluminum.

The invention may best be understood by recourse to 3,725,057 Patented Apr. 3, 1973 FIG. 1 of the drawing wherein relative as-cast grain size as the ordinate is plotted against a parameter P as the abscissa for a number of alloys of nickel. With respect to the axis of ordinates, the value 1.0 is assigned to the average as-cast grain size of pure nickel which was cast under the conditions subsequently set forth. The values less than unity represent the relationship of measured smaller grain sizes to the as-cast grain size of the pure metal as determined by the fractional expression R /R wherein R is average measured grain size of a dilute alloy having a relatively smaller grain size and R is the measured grain size of the pure metal. Thus, for example, the average measured grain size of the pure cast nickel, R was about 3.7 mm. and that of the alloy nickel plus 1 atomic percent chromium was about 1.88 mm. The fraction of these grain sizes equals the relative grain size 0.508 or about 0.51, as plotted. With respect to the abscissa parameter expression m is the slope of the binary phase diagram liquidus line, K is the solute distribution coefiicient, and C is the bulk composition. The solute distribution coefficient, K is the ratio of the solute composition of the solid to the solute composition of the liquid at an appropriate temperature. These values are determinable from a typical binary equilibrium phase diagram as follows. A portion of a hypothetical binary phase diagram is shown in FIG. 2 wherein, as conventional, the plot includes temperature in degrees Celsius as the axis of ordinates and the composition in atomic percent of the constituents as the axis of abscissas. Line L is the liquidus line and line S is the solidus line, e.g., for any given composition, all material at temperatures above L is liquid, all material below S is solid, all material at temperatures between L and S is a mixture of liquid and solid materials and T is the melting point of the pure base metal. Using such a binary diagram, the values of the parameter are determined as follows. Assume the parameter values are to be determined for an alloy having the composition X atomic percent alloy addition and (-X) atomic percent base metal, shown by line C. In terms of the parameter P, X atomic percent is equal to C The parametric slope m is that of a straight line drawn tangentially to the liquidus line at the intersection of lines C and L, as shown. The composition of the solute in the solid and the liquid of the assumed alloy is found at the points C and C respectively, in terms of atomic percent of the alloy addition at the temperature T At the temperature T the ratio of C /C equals the solute distribution efiicient K, of the parameter P. When the composition of the alloy involves very dilute solutions, i.e. where C is of the order of 1.0 or less, satisfactory approximations of parameter values may be made by assuming linear relationships for lines L and S and for slope m For example, the slope m may be taken as the slope of the liquidus line L at its point of origin as shown by line m and an arbitary temperature T which is less than T is selected. When such a approximation is made, the closer T is to T the more accurate the approximation will be. It will be appreciated, however, that the degree of accuracy attainable by this technique will depend upon the accuracy of the phase diagram plot and the degree of linearity of those portions of the liquidus and solidus lines under consideration. In this connection, it will be noted that certain binary systems exhibit almost perfect linearity in these zones, for example, the phase diagram for the aluminumcopper system. Furthermore, an extremely high degree of compositional accuracy is not needed, nor, in fact, is it obtainable in conventional, useful melting and alloying practice.

The curve illustrated in FIG. 1 was determined in the following manner. Two pound castings composed of substantially pure (about 99.99) nickel and nickel containing about 1 atom percent cobalt, aluminum, chromium, copper, silicon, germanium, indium, tin, lead, bismuth and cerium were prepared as follows. Substantially pure nickel was melted in a clean ceramic crucible under a positive pressure of argon in a closed system. The argon was pumped out while the nickel was still molten and the melt permitted to solidify under a vacuum. The nickel was then remelted under argon and where appropriate, the alloy additions made. The various melts were each then poured with a 50 C. superheat into a two inch diameter by four inch copper mold and solidified therein under identical conditions. The castings were cut axially and examined metallographically. The as-cast average grain sizes of each of the several specimens were determined by the straight line intercept counting technique and the corresponding relative grain sizes determined, and the parametric factors and the corresponding values for P were determined as follows:

Plotting the relative grain size values versus the parameter values from the foregoing table produced the exponential curve shown by FIG. 1, the values for m;, and K being determined from published binary phase diagrams. It is noted that the parameters for Ce and Bi alloys were large (approaching infinity) due to the extremely small values of K (approaching and hence were not plotted in FIG. 1.

A similar set of castings were made using high purity (about 99.993 percent pure) aluminum and such aluminum alloyed with about one atomic percent each of lithium, zinc, silver, magnesium, copper, silicon, calcium, cerium, indium, nickel, germanium and tin. The pure aluminum and these aluminum-base alloys were melted in vacuum in high purity alumina crucibles and the alloying and pouring operations were performed under a positive pressure of argon except in the case of the silver alloy wherein the silver was added in vacuo. The same kind of mold was used as in the nickel-base alloys, namely two inch diameter by four inch copper. Again, the castings were sectioned and the average as-cast grain sizes determined in the same manner as for the nickelbase alloys. The values of P for each of these alloys was determined by determining the values of the parameter from published phase diagrams and are listed as follows,

1 Approximately 0. 1 Greater than 70.

As before, the measured grain sizes of the several castings were plotted versus the parametric values P for each as shown in FIG. 3. It should be noted that the parameters for the cerium, indium, nickel, and tin alloys were very large (approaching infinity) due to the extremely small values of K (approaching 0) and hence not plotted. The value of K, for the germanium alloy is open to question because of the uncertainty of the so-lidus line in the binary phase diagram and therefore has not been plotted. The grain size of this alloy was also very small.

Similar grain size determinations were made upon castings of substantially pure nickel containing five atomic percent of the allowing elements listed in Table I. The value of the parameter for each of these alloys is of course five times that for the corresponding one atomic percent alloys. The relative grain size values for these alloys closely approximated the values predicted from the curve of FIG. 1.

The use of the parametric relationship to as-cast grain size may be illustrated as follows. Assume, under a given set of casting conditions, a nickel casting has an average grain size R and it is desired to reduce the grain size thereof to about 30 percent of that value selecting tin as the alloying constituent. From the curve of FIG. 1, the value of P corresponding to a relative grain size of 0.3 is about 6. From the known values of m;, and K for the binary alloy phase diagram, the parametric relationship rs Thus, C =4 atomic percent Cr to be added.

It will be apparent that the same sort of application of the parametric relationship may be used for other base metals and alloys.

In more complex alloy systems than binary the value of the parameter P of as-cast grain size may also be employed similarly to refine the grain size, although not nearly as quantitatively as in the binary systems. In such complex alloy systems, in using the parametric relationship of this invention, a grain refining solute metal is selected on the basis of the numerical value of the parameter P, the formula data being based on the binary phase diagram of the selected grain refining solute metal and the solvent metal of the alloy system. Where the parameter P, so calculated, has a numerical value greater than 10, then that selected metal will cause grain refinement to take place in the alloy system under consideration. For example, by using a commercial nickel-base superalloy having the nominal composition of 61.55% Ni, 9.6% Cr, 4.2% Ti, 5.5% Al, 15.0% Co, 3.0% M0, 1.0% V, 0.15% B, 0.07% Zr and 0.18% C, all by weight, 1 atomic percent cerium was added, and the parameter P was calculated as though the solvent metal of the alloy system was pure nickel. The parameter P thus calculated was very large, namely greater than 250, thus indicating that the selected solute metal cerium would refine the grain. A resulting reduction in as-cast grain size of about 20% was achieved with the added 1 atomic percent cerium, as compared to a similar casting in which the cerium was omitted.

From all the foregoing, it will be apparent that by using the published phase diagram information and the disclosed relationship between the relative grain size and the parameter P, controlled as-cast grain size may be achieved. It will also be apparent thatthe algebraic sign of the slope m is significant and that the value for P in each case is a positive real number. Thus, by solving the equation for theatomic percent value, C the identity of a selected solute metal additive to achieve a smaller grain size is determined. The magnitude of grain refinement may be determined quantitatively in the case of simple binary systems.

Although the examples disclosed above relate to metals and alloys, it should be understood that the principles disclosed herein can also apply to other materials which solidify to form crystals, such as certain ceramics.

Furthermore, the equations set forth are reversible and can be used to determine the identity of ingredients which will coarsen grain size, rather than refine it. Thus, the method set forth in this disclosure may be utilized to determine the identity of impurities which are detrimental to achieving very coarse grain sizes. Also, in columnar growth during directional solidification, a proportionately coarser grain size may be desired, and the method according to the present invention can be used to determine which impurity elements should be reduced to coarsen the structure.

Moreover, although binary phase diagrams are referred to in determining known data for finding the slope of the liquidus line, analogous data for ternary systems and more complex alloy systems may be determined by approximation from known binary phase diagrams, using information related to the major constituents contained in the complex alloy, and making approximations from such data to plot on the exponential curves, and to use in the formula.

It will be obvious to those skilled in the art upon reading the foregoing disclosure that many modifications and alterations in the specific method steps and nonlimiting examples referred to may be made within the general context of the invention, and that various alterations and additions may be made thereto within the true spirit and scope of the invention as set forth in the appended claims.

What we claim as new and desired to secure by Letters Patent of the United States is:

1. The method of producing an alloy casting comprising a solvent and a grain-refining solute element having a predetermined average grain size smaller than that of a casting consisting of the solvent, which comprises the steps of: v

(a) measuring the slope of the liquidus line of the binary phase diagram of the solvent and a selected grain-refining solute element;

(b) measuring the proportion of grain-refining solute element in solid phase to that in liquid phase of the binary system of step (a) at a temperature below the melting-point temperature of the solvent;

() measuring the grain size of each of a plurality of single-phase binary alloy castings of the solvent and a variety of grain-refining-solute elements in proportion such that single-phase solid is in equilibrium with single-phase liquid as the castings are formed;

((1) plotting the thus measured grain sizes as relative grain sizes against a parameter P as the product of the numerical value of said liquidus line slope and the numerical value of atomic percent of each said solute element of each respective binary casting multiplied by the fraction unity minus the numerical value of the proportion measured in step (b) divided by the numerical value of the said measured proportion;

(e) ascertaining from the resulting exponential curve the numerical value of said parameter P corresponding to said predetermined grain size;

(f) determining the quotient of the product of the numerical value of said parameter P and said measured proportion of step (b) as the dividend and the product of the numerical values of the said liquidus line slope and unity minus the said measured proportion of step (b) as the divisor;

(g) adding to the solvent said selected grain-refining solute element in the atomic percent amount of the numerical value of the quotient of step (f);

and finally,

(h) casting and solidifying the resulting alloy.

2. Method according to claim 1, wherein said grainrefining-solute elements in said binary alloy castings of solvent are present in amounts of from a trace to 5 atomic percent.

3. Method according to claim 1, wherein said solvent is nickel and said selected solute element is tin.

4. Method according to claim 1, wherein said solvent is predominantly nickel and said selected solute element is cerium.

5. Method of producing a casting of an alloy comprising a solvent element, a plurality of alloying additions including at least one grain-refining solute element and having an average grain size smaller than that of a casting excluding said grain refining solute elements, which comprises the steps of:

(a) determining the numerical value of parameter P in the equation wherein, from a binary phase diagram of said solvent element and a selected grain-refining solute element,

m is the liquidus line slope; K is the solute distribution coefficient; and C is the bulk composition in atomic percent of said selected grain refining solute element; (b) said one grain refining element being selected so that said parameter P will have a numerical value significantly greater than 10, and

(0) adding to a melt of said alloy a quantity of said selected grain refining element corresponding to the value C in atomic percent; and

(d) casting and solidifying the resulting alloy.

6. Method of coarsening the grain size of an alloy casting containing a solvent element and solute elements, comprising the steps of:

(a) determining the numerical value of parameter P in the equation C is the bulk composition in atomic percent of References Cited each of the respective solute elements; UNITED STATES PATENTS (b) selecting those solute elements having a numerical value of P significantly greater than 10, 2,280,169 4/1942 Stroup 75 138 (c) making a melt with a concentration of the solute 5 elements selected in step (b) reduced so that the RICHARD DEAN Pnmary Exammer numerical value of P is significantly less than 10, Us Cl XR and (d) casting and solidifying the resulting alloy. 147, 1 

